perf: linear content accumulation in the streaming response handler (#27231)

The streaming handler rebuilt the full accumulated response with
`content = f'{content}{value}'` on every content delta — a complete
string copy per chunk, making accumulation O(n^2) over the response
length. Use in-place `content += value` for the (universal) str case,
which CPython extends in place, keeping accumulation O(n); the
f-string fallback is preserved for non-str values so coercion
behavior is unchanged.

In the ENABLE_REALTIME_CHAT_SAVE branch, full_output() — which
concatenates the entire accumulated output — was called twice per
chunk (once for the DB upsert, once for the emitted delta). Compute
it once and reuse.


Claude-Session: https://claude.ai/code/session_01MHg5zs1VBjvRWQ54qHpfYD

Co-authored-by: Claude <noreply@anthropic.com>

backend/open_webui/utils/middleware.py

@@ -4490,7 +4490,12 @@ async def streaming_chat_response_handler(response, ctx):
                                                 user,
                                             )
 
-                                        content = f'{content}{value}'
+                                        if isinstance(value, str):
+                                            # In-place append — avoids copying the full
+                                            # accumulated response on every chunk.
+                                            content += value
+                                        else:
+                                            content = f'{content}{value}'
 
                                         # Check if we're inside a tag-based block
                                         # (reasoning, code_interpreter, or solution).
@@ -4597,16 +4602,17 @@ async def streaming_chat_response_handler(response, ctx):
                                         if ENABLE_REALTIME_CHAT_SAVE and not metadata.get('chat_id', '').startswith(
                                             'channel:'
                                         ):
+                                            current_output = full_output()
                                             # Save message in the database
                                             await Chats.upsert_message_to_chat_by_id_and_message_id(
                                                 metadata['chat_id'],
                                                 metadata['message_id'],
                                                 {
-                                                    'output': full_output(),
+                                                    'output': current_output,
                                                 },
                                             )
                                             data = {
-                                                'output': full_output(),
+                                                'output': current_output,
                                             }
                                             delta_type = 'content'
                                         else: